Horton

Re: [HACKERS] logical decoding of two-phase transactions

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: Masahiko Sawada <sawada.mshk@gmail.com>
Cc: "tanghy.fnst@fujitsu.com" <tanghy.fnst@fujitsu.com>, vignesh C <vignesh21@gmail.com>, Peter Smith <smithpb2250@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>, Ajin Cherian <itsajin@gmail.com>, Greg Nancarrow <gregn4422@gmail.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2022-01-05T02:51:52Z
Lists: pgsql-hackers 
On Tue, Jan 4, 2022 at 9:00 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> According to the doc, the two_phase field has:
>
> True if the slot is enabled for decoding prepared transactions. Always
> false for physical slots.
>
> It's unnatural a bit to me that replication slots have such a property
> since the replication slots have been used to protect WAL and tuples
> that are required for logical decoding, physical replication, and
> backup, etc from removal. Also, it seems that even if a replication
> slot is created with two_phase = off, it's overwritten to on if the
> plugin enables two-phase option. Is there any reason why we can turn
> on and off this value on the replication slot side and is there any
> use case where the replication slot’s two_phase is false and the
> plugin’s two-phase option is on and vice versa?
>

We enable two_phase only when we start streaming from the
subscriber-side. This is required because we can't enable it till the
initial sync is complete, otherwise, it could lead to loss of data.
See comments atop worker.c (description under the title: TWO_PHASE
TRANSACTIONS).

> I think that we can
> have replication slots always have two_phase_at value and remove the
> two_phase field from the view.
>

I am not sure how that will work because we can allow streaming of
prepared transactions when the same is enabled at the CREATE
SUBSCRIPTION time, the default for which is false.

-- 
With Regards,
Amit Kapila.

Commits

  1. Add prepare API support for streaming transactions in logical replication.

  2. Doc: minor improvements for logical replication protocol documentation.

  3. Fix test failure in 021_twophase.pl.

  4. Refactor to make common functions in proto.c and worker.c.

  5. Unify PostgresNode's new() and get_new_node() methods

  6. Fix potential buffer overruns in proto.c.

  7. Add support for prepared transactions to built-in logical replication.

  8. Refactor function parse_subscription_options.

  9. Allow enabling two-phase option via replication protocol.

  10. Don't use Asserts to check for violations of replication protocol.

  11. Improve psql tab completion for options of subcriptions and publications

  12. Rearrange logrep worker's snapshot handling some more.

  13. doc: Update information of new messages for logical replication.

  14. ALTER SUBSCRIPTION ... ADD/DROP PUBLICATION

  15. Allow pgoutput to send logical decoding messages.

  16. Refactor function parse_output_parameters.

  17. Avoid repeated decoding of prepared transactions after a restart.

  18. Fix an oversight in ReorderBufferFinishPrepared.

  19. Allow multiple xacts during table sync in logical replication.

  20. Fix replication of in-progress transactions in tablesync worker.

  21. Fix 'skip-empty-xacts' option in test_decoding for streaming mode.

  22. Use Enum for top level logical replication message types.

  23. Add support for streaming to built-in logical replication.

  24. Fix the logical streaming test.

  25. Implement streaming mode in ReorderBuffer.

  26. Extend the logical decoding output plugin API with stream methods.

  27. Store 2PC GID in commit/abort WAL recs for logical decoding